Electron focusing system



y L. F. BROADWAY 2,283,041

ELECTRON FOCUSING SYSTEM Filed Aug. 30, 1940 INVENTOR LEONAROFRANC/S BROADWAY ATTORNEY Patented May 12, 1942 ELECTRON FOCUSIN G SYSTEM Leonard Francis Broadway, Ickenham, England,

assig'nor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application August 30, 1940, Serial No. 354,882

In Great Britain September 7, 1939 3 Claims.

This invention relates to electron focusing systems used in electron discharge tubes such as cathode ray tubes.

The electron gun in a cathode ray tube usually comprises a source of electrons such as a directly or indirectly heated cathode, a shield surrounding the cathode, which may also serve as a modulating electrode, and first and second anodes or accelerating electrodes between which an electron lens is formed which focuses the electrons upon a screen such as a fluorescent screen or a mosaic electrode. Electro-magnetic focusing means such as a coil surrounding the electron beam may be employed instead of electrostatic means.

Hitherto it has been customary in designing electron guns to utilise an electrode system which causes the electron beam proceeding from the cathode surface to cross over in the vicinity of the cathode. The beam diverging from this cross-over may then be converged on to the fluorescent screen by the final lens of the gun. In the case of a more complicated electrode system the beam may be caused to cross-over a second time before being finally focused. In general, the operating conditions of the tube are so chosen that the sharply focused spot formed on the fluorescent screen of the cathode ray tube is 7 an image of the electron cross-over point; oc-

casionally, however, the image of an aperture placed near this cross-over point is used instead.

In a typical known form of electron gunthe electron beam is caused to cross over at the cathode by means of the converging lens formed by the penetration of the first anode field through the apertured electrode which serves to modulate the beam. After leaving the cross-over the beam is defined by a stopping diaphragm, and finally focused on the fluorescent or other screen by the electrostatic lens formed between the first and second anodes.

It has been shown by electron optical investigations that the real cross-over which is used in such a case, has considerable disadvantages, particularly in the case of tubes designed for operation at relatively low voltages and high current densities. It appears that the current density obtainable in the focused electron beam of such tubes is limited by the mutual repulsion of the electrons at the cross-over point rather than by the laws of electron optics.

The object of the present invention is to provide an electron gun and a method of operation therefor, which substantially reduces, or elimito the cause mentioned, and an essential feature of the invention lies in designing an electron gun such that the image of a virtual cross-over'instead of a real cross-over is produced upon the screen.

According to the present invention the electron gun in a cathode ray tube comprises acathode, a modulating electrode and first and second anodes and saidcathode has a convex emitting surface, the portion of said modulating electrode adjacent the'emittingsurface of said cathode being composed of fine wire mesh shaped to conform substantially to the shape of said emitting surface, whereby penetration of the anode field within said modulatingelectrode is substantiallyprevented and said electron gun, in operation, produces an electron beam in which no cross-over occurs between said emitting surface and the point at which it is desired to focus said beam.

In order that the invention may be more clearly understood and readily carried into effect, one form of electron gun designed and operated in accordance therewith, will now be described by way of example with reference to the accompanying drawing.

Referring to the drawing, the electron gun includes a convex cathode C which is indirectly heated by a filament F, a modulating electrode M and the first and second anodes A1 and A2 respectively. The modulator electrode M consists of a fine gauze mesh which is carefully formed so that it is always parallel to the convex emitting surface of the cathode, and the mesh of which is so fine that substantially no penetration of the anode field takes place in the region between the mesh and the cathode. It has been found that a gauze containing more than 100 meshes per inch is satisfactory for this purpose. In these circumstances the potential of the modulator electrode at the point at which the electron beam current is cut off, is only slightly negative with respect to the potential of the cathode, and in operation the modulation voltage is mainly applied in the positive sense with respect to the cathode potential. While 1 have shown the tube in the drawing in section, it will be appreciated that the anodes A1 and A2 are cylindrical and that the effective surfaces of the convex cathode C and control or modulator electrode M are substantially spherical in form and are concentric one with the other.

The electron beam diverges from the convex surface of the cathode, the outline of the beam nates, the limitations of the current density due being indicated at B, and is brought to focus on a fluorescent screen by means of the electrostatic lens formed between the first and second anodes A1 and A2. Alternatively, the beam may be focussed by electro-magnetic means. In the case of electrostatic focusing the potential of the anode A1 may be several hundred volts, and the potential of the anode A2 may be several thousands of volts, but it will of course be understood that these voltages will be determined by the general design of the tube. It will be seen from the drawing that the electron beam has only one real cross-over point, namely, at its final focus upon the target such as the fluorescent screen S, the other cross-over being virtual and situated behind the cathode surface at the point P.

It Will be seen from comparison between known forms of electron gun and that illustrated in the drawing, that the desired result has been accom-- plished by removal of the converging lens formed by thepenetration of the first anode field through the aperture of the modulator electrode and the substitution of a divergent system formed by the cathode surface and the modulator electrode. It will be understood that the same result may be attained by other arrangements than the precise arrangement shown in the drawing.

I claim: V

l. A cathode ray tube comprising an envelope, a target within said envelope, a convex cathode facing and exposed to said target, a controlelectrode adjacent said cathode having a portion effective on electrons emitted by said cathode which is concentric with said convex cathode 1 whereby electrons aredirected toward said target along diverging paths, said cathode and com trol electrode having centers of curvature more remote from said target than the convex surfaces thereof, and means to terminate said diverging paths and direct said electrons along converging paths to said target.

2. A cathode ray tube comprising an envelope, a target within said envelope, a cathode having a spherical surface whose center of curvature-is more remote from said target than said surface, a modulator electrode of fine wire mesh gauze adjacent and concentric with the spherical a fine wire mesh of at least one hundred mesh per inch interposed betweensaidcathode and anode the surface of said mesh wbeing convex toward said target to prevent substantial penetration of the field generated byv said anode within the space betweensaid cathode and electrode and to .cause electrons emitted bysaid cathode to diverge from an axis between said cathode and target and means to direct electrons following diverging paths along converging paths substantially intersecting at said target.

7 LEONARD FRANCIS BROADWAY. 

